1. Technical Field
The present invention relates to the field of computer data storage systems, and particularly to storage systems (for example, block storage controllers for a storage area network) requiring extended scalability.
2. Description of Prior Art
Modern storage systems provide increasing storage capacity to meet user demands from host computer system applications. Various storage device configurations are known and used to meet the demand for higher storage capacity, performance and availability.
An exemplary configuration according to the art is shown in FIG. 1. As shown in FIG. 1, one of the storage configurations that may be used to meet demands for increased capacity and reliability is the use of multiple smaller storage modules which are configured to ensure data integrity and continuous availability in case of failures. In such redundant systems, recovery from many types of failure can be automated within the storage subsystem itself due to the use of redundancy. An example of such a redundant subsystem comprises dual controllers with redundant arrays of inexpensive disks (RAID), shown in schematic form in FIG. 1 as 112 and 114.
Typical storage systems have two storage device controllers 108, 110, that manage the storage and provide upstream interfaces for I/O to the storage. The storage device controllers may manage, for example, an array of storage devices for one or more host systems. The storage controllers may be used to make the arrays of storage devices 112, 114, appear to a host system, 102, 104, to be a single, high capacity storage pool.
Many modern storage systems are configured in storage area networks with access to storage by host computers 102, 104, being controlled across communications links. A storage area network (SAN) is a network whose primary purpose is the transfer of data between computer systems, such as hosts 102, 104, and storage elements located in storage subsystems, for example, storage devices 112, 114. In a SAN, storage devices may be centralised and interconnected in a high-speed network that allows the establishment of communications between storage subsystems and host computers within the distance supported by the communication infrastructure. A SAN can be shared between hosts or dedicated to one host. It can be local, or can be extended over geographical distances.
SANs enable storage to be externalised from the hosts and centralised elsewhere. This allows storage and data to be shared among multiple hosts. Data sharing enables access of common data for processing by multiple computer platforms or hosts.
The host infrastructure of a SAN can include a mixture of host platforms. The storage infrastructure includes storage devices which are attached to the SAN network. SANs can interconnect storage interfaces together into many network configurations.
A typical SAN uses what is known as a fabric 106 to connect devices. A fabric is the term used to describe the infrastructure connecting hosts and storage subsystems using interconnect entities such as switches, routers, directors, hubs and gateways. The different types of interconnect entities allow networks of varying scale to be built.
Turning to the storage controllers, 108, 110, it is well known in the art for conventional controllers to comprise cache memories, in order to take advantage of electronic access speed. Such caches may be implemented, for example, in DRAM technology. In operation, the caches 116, 118, provide a temporary store for data associated with I/O requests received by controllers 108, 110.
The direct association of caches and I/O ports with individual controllers is typically a limiting factor in achieving scalability in the performance, capacity and host connectivity of a storage subsystem, and it would thus be desirable to provide an arrangement, apparatus, method or computer program which would alleviate this problem.
It is known to have pairings of controllers for higher availability with capability to access data through either controller, but these systems are inherently limited in the scalability of their capacity and performance. It would thus be further desirable to overcome this limitation.